The invention concerns apparatus for separation of substances by permeation of liquids through porous adsorption membranes.
Adsorptive separation in the context of the present invention means separation or purification of specific substances from a liquid phase medium, which are specifically adsorbed by a solid adsorbent. The medium containing the substances to be separated and/or purified is fed to or through the solid adsorbent whereby adsorption takes place, then separated by means of one or more elution liquids (eluants) that are forced through the adsorbent under pressure. Depending on the degree of interaction between the substances in the medium with the solid adsorbent and with the eluants, the substances are retained with different strength by the adsorbent, resulting in fractionation of the substances from the adsorbent. The substances to be separated from the medium can either be adsorbed alone or together on the adsorbent. In the latter case, the medium is filtered with the target mixture to be separated through the adsorber module, until the target-depleted substance appears at the outlet of the module. It can be eluted separately from other substances retained on the adsorbent with appropriate eluants that are flowed through the module, referred to as stage elution. Undesired substances or contaminants may also be separated from the medium.
Another area of application of adsorbtive separation by the present invention is chromatography, wherein only part of the adsorption capacity of the adsorber is utilized for adsorption and separation of the adsorbed components is conducted using different volumes of eluant for elution.
The interactions between solid and liquid phases, therefore, play an important role in adsorptive separation, wherein the solid phase must exhibit a high specific affinity surface to achieve high efficiency, and therefore must have either limited particle size or high porosity. Since limits are imposed on the use of extremely fine solids, in practice highly porous matrices are generally used as solid phases. The use of porous matrices means that the kinetics of the fundamental process of adsorption/desorption, i.e., the interaction between the components of the liquid phase with the solid phase, is superimposed on the kinetics of mass transport into and out of the porous matrix. Since mass transport in known matrices (as in particulate matrices) primarily occurs by diffusion, and since diffusion coefficients of liquids tend to be low, there is an inherent limitation on the efficiency of this type of adsorptive separation.
On the other hand, nonparticulate matrices having continuous pore structures such as porous membranes offer the possibility of primarily convective mass transport under a pressure differential, thus eliminating the aforementioned diffusion limitation on efficiency.
In the context of the present invention, the term "adsorption membrane" broadly means membranes that carry functional groups, ligands or reactants on their internal and/or external surfaces that are capable of interaction with at least one substance of a liquid phase in contact with them. Exemplary types of adsorption membranes include cationic, anionic, ligand, affinity or other activated membranes that in turn may be classified according to their particular functional groups, ligands or reactants.
The term "porous adsorption membrane" refers to membranes whose average pore diameter preferably lies in the microfiltration range, e.g., between about 0.1 .mu.m and about 15 .mu.m. The thickness of porous adsorption membranes preferably lies between about 100 .mu.m and about 500 .mu.m.
Methods and apparatus for adsorptive separations by permeation of liquids through porous adsorption membranes are known. See, for example, U.S. Pat. No. 5,575,910, wherein a combination of pressurized axial and radial feeds are used. To increase adsorption capacity, a plurality of membrane adsorption packs, each formed from a number of planar sheet of porous adsorption membranes, are used in a stacked arrangement with spacers between each pack. Although efficient separation is achievable with such a device, it has a drawback in that fabrication results in significant waste of valuable adsorption membrane material. Moreover, an increase in the number of planar sheets of porous adsorption membranes is associated with a reduction in flow performance and relatively rapid blinding of the upper layers of the adsorption membranes packs.
Use of spiral wound adsorption material is known from U.S. Pat. Nos. 4,895,806 and 4,986,909, this form tending to minimize waste of valuable adsorption membranes. Both patents disclose a sheet of adsorption membrane wound onto a perforated central tube which filters by forcing the liquid medium from the outside radially inwardly by application of a pressure differential. A shortcoming of such designs is that the permeation-effective surface in a winding tends to diminish from the outside in. Furthermore, if the diameter of the perforated central tube is small, the flux of the device is correspondingly small; on the other hand, if the tube diameter is large, the dead volume of the device is also large. Such spiral wound designs have the further drawbacks that the flow performance diminishes rapidly and filter lifetime is relatively limited, owing to membrane fouling and defects. Yet another drawback of such spiral wound designs is that they possess a rigid configuration that does not permit flexible adaptation to a variety of separation tasks.
The aim of the present invention is therefore to devise an apparatus, configured with an adsorber module, to conduct adsorptive separations by permeation of liquids through porous adsorption membranes, the apparatus being characterized by minimal dead volume, optimal active adsorption filtration volume with high binding capacity, high flux and long lifetime, as well as by a high degree of flexibility for adaptation to a wide variety of separation tasks. A related goal of the present invention is to propose applications for the use of inventive device for adsorptive substance separations.
These goals and others which will become apparent to those skilled in the art are achieved by a uniquely designed device having the function and adaptability summarized and described in detail below.